Transistors in vast quantities linked into massive integrated circuits form the backbones of microprocessors and other high speed active electronic devices. As the maximum available processing speeds of such devices continually grow, the demand for transistors facilitating still greater processing speeds likewise grows unabated. A fundamental strategy for delivering such ever increasing speed involves progressively scaling down the transistor dimensions.
Heterobipolar transistors (“HBTs”) are one common type of transistors. HBTs are fabricated from P and N type doped semiconductor materials. N type doped semiconductor materials comprise excess free electrons, and P type doped semiconductor materials comprise excess free holes. N type doped semiconductor materials can conduct an electrical current by the transfer of free electrons, as they are doped with donors of negative charge carriers. P type doped semiconductor materials can conduct an electrical current by the transfer of free holes, as they are doped with negative charge acceptors.
In one type of HBT, referred to as an NPN transistor, the basic active structure comprises two layers of an N type doped semiconductor with a layer of a P type doped semiconductor sandwiched in between. The middle layer receives an input signal to be amplified or switched, and is referred to as the base. One of the two outside layers receives an electrical power input to the HBT and is referred to as the emitter. The other of the two outside layers provides an electrical power output from the transistor and is referred to as the collector. The voltage of the base must be more positive than that of the emitter, and the voltage of the collector must be more positive than that of the base. Modulation of an electrical signal that is input to the base controls the output signal to the collector. A small current that is input to the base can control a much larger current flowing from the emitter to the collector.
In another type of HBT, referred to as a PNP transistor, the basic active structure comprises two layers of a P type doped semiconductor with a layer of an N type doped semiconductor sandwiched in between. The middle layer again receives an input signal to be amplified or switched, and is referred to as the base. One of the two outside layers receives an electrical power input to the HBT and is referred to as the emitter. The other of the two outside layers provides an electrical power output from the transistor and is referred to as the collector. The voltage of the base must be more positive than that of the collector, and the voltage of the emitter must be more positive than that of the base. Modulation of an electrical signal that is input to the base controls the output signal to the collector.
A negative current flows from the emitter to the collector in an NPN transistor, whereas a positive current flows from the emitter to the collector in a PNP transistor. Electrons typically travel more rapidly than holes. Accordingly, NPN transistors are generally preferred, particular in applications for carrying signals at frequencies in excess of 1 gigahertz (“Ghz”). However, PNP transistors, and combinations of PNP and NPN transistors, can also be used in various end use applications.
HBT structures include one or more heterojunctions. A heterojunction is defined as an interface between two semiconductor materials having different compositions. In addition or alternatively, the two materials may be of different conduction types, that is, N or P type conductors. Common types of heterojunctions for HBTs include AlGaAs/GaAs, InGaP/GaAs, InP/InGaAs, and InGaAlAs/InGaAs, where the “/” indicates an interface. The heterojunction leads to the formation of a potential barrier in either the conduction band or the valence band, blocking the flow of one type of carrier while allowing the flow of carriers having the opposite charge. For example, in an N—InP/P—InGaAs emitter-base HBT junction, electrons are permitted to flow from the emitter into the base in forward bias, while holes are blocked from entering the emitter from the base. This heterojunction design typically results in a high injection efficiency of electrons over holes, and in higher device speed as compared to a classic bipolar transistor, since electrons move much faster than holes in this material system.
Electrodes are separately placed in contact with each of the three semiconductor layers in the HBT devices. Hence, a portion of the base semiconductor layer is exposed so that a base electrode can be applied to that layer. Although the three semiconductor layers are sandwiched in mutual contact, the three electrodes need to be mutually isolated to prevent shorting of the HBT. Hence, for example, a suitable distance between the base electrode and the emitter layer is maintained. In a typical conventional HBT transistor, the minimum allowable distance between the base electrode and the emitter layer is about 2000 Angstroms (Å). The base access resistance is given by the product of base sheet resistance and the separation described above. The sheet resistance of the base layer is typically within a range between about 200 Ohms (Ω) per square (sq) and about 1000 Ω/sq.
Efforts to scale down the size of HBT transistors desirably involve reducing the thicknesses of the semiconductor layers as well as the length and width dimensions of features in such layers. Such size reductions reduce the distances separating the electrodes and semiconductor layers for the emitter, collector and base, thereby increasing the potential for electrical shorting. Hence, for example, transistor designs have typically required the maintenance of a defined distance between the base electrode and the emitter layer.
There is a continuing need for HBTs characterized by high speed and small dimensions that are not typically subject to shorting, and for integrated circuits comprising such HBTs. There further is a need for suitable processes of making such HBTs.